# Full text of "Text Book Of Mechanical Engineering"

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```Appendix  VI.                          1179

in opposite directions eliminates axial pressure, as in Parsons'
turbine at a Fig. 1034. Also in marine work the two turbines may
be so proportioned as to resist the thrust of the propeller itself.

Dynamos driven by turbines are specially designed, and 1500
revs, per m. are recommended. Marine propellers are also built
of small diameter, and with a pitch suitable to the high speed of
revolution.

Steam Turbine Calculations.—It is required to find the
cross-sectional area of steam fluid as it expands adiabatically
without frictional resistance. As there is no friction the expansion,
results in a decrease of pressure and increase of velocity such that
the pressure energy of the source is entirely converted into kinetic
energy when the steam pressure is zero. We shall consider the
particular case of steam at 165 Ibs. absolute pressure, expanding
down to i Ib. absolute, and the changes may occur either in the
length of the de Laval nozzle, or through the wheels of a many-
stage turbine, the stream areas found being the cross-sectional
areas of the nozzle or blade spaces. Let suffix i indicate
commencement^ and suffix 2 any other position of the stream, and
all calculations be per Ib. wt. of steam.

Energy at pressure PJ.......= J (Si + L^)

Energy at any other pressure P2 = J (S2-f#L2)
where x is-the dryness fraction.

The stream becomes wet as it parts with its internal energy,
and the value of x can be found from the r0 diagram, Fig. 1035.
Firstly, construct the diagram on a large scale by the method of
pp. 887-8. Next draw line k h at temperature 366° F. for 165 Ibs.
pressure, and drop perpendicular h m. As explained at Fig. 851,
.p. 890, the part ac is dry steam, and cb the condensation. A
series of horizontals are drawn to represent pressures (a varying
scale), and the values of a b and a c are shewn in columns /
and g respectively by entropy units. Then—

•r^          /•     •               ac      £

Dryness fraction x =   -r = %•

and the results are shewn on the/ight Deducing from pp, 887-8,
the following will represent the method algebraically :—

if - "*< 3```